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Recap – Covalent Bonding• Electrons are shared, bonds are

directional.• Valence electrons are outer shell

electrons.• Valence of the element give No of

bonds:– 1 H, F, Cl, Br, I– 2 O– 3 N– 4 C CH

O

C N

double bond single bond triple bond

lone pair

bond pair

Max no. of valence electronsFirst shell holds a maximum of 2 electrons• Hydrogen – max of 2 electrons in bond

Second shell holds a maximum of 8 electrons• C, N, O, F – max of 8 valence electrons /

atom

Third shell holds a maximum of 18 electrons• P, S, Cl – may have 8, 10, 12 valence

electrons / atom.2

Example - phosphorus

3

P

Core electrons: 2 + 8(not shown)

Valence electrons: 5

XX

X

X X

F P F

F

F P

F

F

F

F

PF5

PF3

Example - sulfur

4

F S F

F S F

F F

F S F

F F

F F

SF6

SF4

SF2

S

Core electrons: 2 + 8(not shown)

Valence electrons: 6

XX

XXXX

X X

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Polyatomic Ions• All compounds so far have been

binary; all ions so far have consisted of single atoms with an electrical charge.

• Some ions, however, have more than one type of atom bound together with covalent bonds – polyatomic ions.

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Polyatomic Ions

H N

H

HH+

H N

H

H

H

Ammonium ion

H

X

H

Hydrogen atom Hydrogen ion H+

remove e-

Note: ‘octet’ of electrons

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Polyatomic Ions

H O H

Hydroxide ion

remove H+

O H

HOX

XXX

X

XX

X X

X add e- (charge)

OX

XXX

X

XX

X HXX

or

Note: ‘octet’ of electrons

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Polyatomic Ions and valence

Note the valence of an element tells us the number of bonds it forms in a neutral molecule. It is most useful when 8 electrons are involved (Period 2). What about polyatomic ions?

In general:• If the charge is negative the atom has one less bond than

expected from its valence.• If the charge is positive the atom has one more bond than

expected from its valence.

HOX

XXX

X

XX

X XH X

H O H

OX

XXX

X

XX

X HXX

HO

HOX

XXX

X

XX

X XH X

H

H O H

H

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Polyatomic Ions – other examples

• Sulfate: SO42-

• Sulfite: SO32-

• Carbonate: CO32-

• Nitrate: NO3-

• Phosphate: PO43-

NamingSee on-line module: http://

firstyear.chem.usyd.edu.au/iChem/inorganic_nomenclature.shtml

In brief: • The element with the lower group no called by

the element name• The name of the other is derived from the

element name and ends in …ide• Use di…, tri…, tetra… as appropriate

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Polar Covalent Bonds• Covalent bonds between identical

atoms have electrons shared equally.

H — H Cl — Cl

If atoms are different, one atom may get the greater share of the electron pair.

Example: HCl

+ -

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Polar Covalent Bonds

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Polar Covalent Bonds

H — H

Cl — Cl

Evenly distributed electrons

Unevenly distributed electrons

-

-+

+

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Polar Covalent BondsExample: H2O

The oxygen end of the molecule is slightly negative relative to the hydrogen ends.

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Bond TypesCl2 Covalent Bond

HCl Polar Covalent Bond

NaCl Ionic BondNa+

Cl-

ClCl

• By the end of this lecture, you should:− recognise elements in Period 2 have 8 electrons

when bonded− elements in Period 3 have 8, 10 or 12 electrons

when bonded− know that polyatomic ions represent two or more

covalently bonded atoms which have an overall charge

− recognise when a polar covalent bond occurs− appreciate bonding represents a range of

situations from even sharing of electrons to complete transfer of electrons

− be able to complete the worksheet (if you haven’t already done so…)

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Learning Outcomes:

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Questions to complete for next lecture:

1. How do you name PCl3 and PCl5 ?(you may need to refer to the naming module http://firstyear.chem.usyd.edu.au/iChem/inorganic_nomenclature.shtml)

2. Chlorine and oxygen react to form the following polyatomic ions: ClO-, ClO2

-, ClO3-, and ClO4

-. Why can chlorine form ions with the same atoms but different formulas?

3. Draw the electron dot structure of the polyatomic ion H3O+ (Hint, review the formation of NH4

+)4. What condition is necessary for a bond to be polar?5. Classify the bonds between the following pairs of

molecules as ‘covalent’, ‘polar covalent’ or ‘ionic’: H and Br; O and O; Mg and Cl; P and O; I and I